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The wonderful world of wetlands and their resilience to climate change

A worldwide study shows that while wetlands are under increasing threat from global warming, rising sea levels and human population density, they're remarkably resilient – and Australia's are faring better than most.

But the research team, led by the University of Lincoln in the UK, found that wetlands could well be more resilient than previously believed. The results have been published in Nature.

The study looked at 12,148 wetland coastline segments around the world, and calculated a “wetland adaptability score”.

The researchers developed an integrated global model taking into account the factors that will influence a wetland’s ability to adapt to a changing environment. These included geographical features, such as cliffs, tidal surges and human population density.

In some parts of the world, rising seas will increase the buildup of sediment in wetlands. In other parts, humans may have to remove sea walls or roads to allow wetlands to migrate – or move – on to land as sea levels rise.

The study found that if wetlands are allowed to move inland, they’ll likely survive. Migration will be particularly important in Australia, Dr Reef says, because the continent is dry, and the amount of sediment deposited by our rivers is relatively small. If the sediment in a wetland doesn’t build up – or accrete – sufficiently, the wetland will be in danger of drowning.

She’s working with the Victorian government to model likely scenarios, as well as strategies to save wetlands.

Wetlands are surprisingly resilient, Dr Reef says, because they’ve had to be.

Since the last ice age, “the world has undergone massive changes in sea level,” she explains.

“Coming out of an ice age as we are, sea levels have been rising for a while. We’ve had periods where sea levels were much higher than today, and periods when they were 140 metres lower. So the coastal zone is very dynamic, and wetlands are habitats that inhabit that very dynamic zone, and they have unique adaptations to try to maintain their position, despite sea levels changing, sediments moving.”

Dr Reef says that as the seas rise, it’s right and proper that the many urban settlements built on former wetlands – including Melbourne’s bayside suburbs – be protected from the tide. “But where there is low population density, there usually isn’t a problem for the wetland to migrate if the topographic conditions are right.”

Important for many reasons

Dr Reef values wetlands because they’re unique and resilient, but she says they’re also important for other reasons.

“They store carbon better than any other vegetated surface that we have. They store the carbon both in the biomass – the mangrove, say - but mostly it’s in the ground, in the soil.”

Wetlands also provide nurseries for fish, sanctuaries for birds, protect the coast against storm surges and prevent erosion.

“The problem is that wetlands occupy a space that we like to occupy as well,” Dr Reef says. “We like to live on the coast. We haven’t been good at giving them their space.”

Since 1700, up to 87 per cent of the planet’s wetlands have been lost – loss that is happening three times faster than the loss of forests.

“But saying that, Australia is doing pretty well compared to the rest of the world,” she says.

Mangroves are protected in Australia, and many of our wetlands are also protected by the international RAMSAR treaty set up by UNESCO in 1971.

“And lucky for us, with sea level rise, our wetlands won’t fare as badly as some places, which is what our modelling shows,” she says.

Land rising, falling

We all know that sea levels will rise when glaciers and ice sheets melt, but less well-known is that land masses are also rising and falling in response – a phenomenon known as post-glacial rebound. It’s been taking place since the last ice age.

Dr Reef explains that ice-bound countries will likely continue to rise up gradually, as the heavy ice disappears from their surface.

This is possible because land masses rest on the upper mantle, or asthenosphere. It’s not molten, but not quite solid, either – it behaves like a fluid over months or years. She makes a see-saw motion with her hand to demonstrate the movement – as land in the north rises, land situated further to the south will sink lower.

“The problem is that wetlands occupy a space that we like to occupy as well. We like to live on the coast. We haven’t been good at giving them their space.”

“So when we think about sea level, we can say that we’re warming the ocean, and we are adding fresh water to the ocean, the sea level is rising – but the other thing is that the continents are moving, they’re tilting back.”

The problem particularly affects the northern hemisphere, she says.

“What we’re seeing now is a rebound from the loss of the ice, from the top, from the north,” she says. “So we see places like southern Europe, parts of Asia and the southern bit of North America actually drowning now, because the continents are tilting back.

“In Australia, we don’t have that problem … We’re actually quite stable, so for us sea levels aren’t rising as quickly as they are at other places around the world.”

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